Coil system for high-frequency heating generators



L. BLOK Oct. 8,Y 1957 COIL SYSTEM FOR HIGH-FREQUENCY HEATING GENERATORS 3 Sheets-Sheetl 1 Filed Dec. 30, 1953 i Ii o1/RENS V51.0K B'Y /CM/ @7 #We AGENT.

Oct. 8, 1957 BLOK 2,809,357

COIL SYSTEM FOR HIGH-FREQUENCY HEATING GENERATORS Filed Dec. 30, 1953 i 5 Sheets-Sheet 2 l 53 49 INVENTOR 8 i 83 bd/'IMS a 7/.70 79 if i.' y 4 MM AGENT Oct. 8, 1957 BLOK 2,809,357

COIL SYSTEM FOR HIGH-FREQUENCY HEATING GENERATORS Filed Dec. 30,' 1953 3 Sheets-Sheet 3 INVENTOR Ol/RENS BLOK AGENT United States Patent COIL SYSTEM FOR HIGH-FREQUENCY HEATING GENERATORS Lourens Blok, Eindhoven, Netherlands, assignor, by mesne assignments, to North American Philips Company, Inc., New York, N. Y., a corporation of Delaware Application December 30, 1953, Serial No. 491,32; Claims priority, application Netherlands January 22, 1953 9 Claims. (Cl. 336-58) This invention concerns coil systems for high-frequency heating generators, which 'comprise an axially slidable water-cooled primary cylinder coil in the form of a helix and a stationary coupling coil surrounding the former coaxially and consisting of a metal strip curved to form a cylindrical shell.

One of the main problems in such devices is to obtain a suiiicient maximum coupling degree between the primary coil and the secondary coil (coupling coil). As a matter of fact, the coupling coil is associated with a Working coil which, similarly to the coupling coil, usually consists of a single turn in whose field the mass to be heated is placed. The impedance formed by the working coil may have very different values and if, in the case of particularly unfavourable values of the impedance, the generator is required to give olf its full power in the working coil, the transfer of this power from the primary coil to the secondary coil will involve the ow of very heavy (wattless) currents which cause considerable losses.

In order to minimize these currents the (maximum) coupling between the primary coil and the secondary coil should be as tight as possible. The present invention has for its object to attain this end with the use of a ferromagnetic core in the primary coil.

The high losses occurring in the core with the use of high frequencies (l to 2 mc./s.) produce considerable heat in the core and the invention has for its main purpose to provide a construction permitting this heat to be carried off in a satisfactory manner.

A further object of the invention is to provide a reliable coil system which occupies little space and permitting of readily attaining the widely varying values of the coupling between primary and secondary coil (coupling coil) in order to cope with different working conditions.

According to the invention these ends are attained by immersing the coil system in insulating oil and by providing in the primary coil a substantially rod-shaped ferromagnetic core of electrically poorly conductive cubic ferrite, the length of the primary winding exceeding more than twice the diameter and the support of the primary winding and the core being provided with apertures in order to permit unimpeded transfer of heat through the oil from the core to the primary winding. As a result of the use of ferrite the losses in the core remain within reasonable limits and the rod-shaped core, in connection with the satisfactory transfer of heat through the oil, permits all parts of the core to be cooled satisfactorily through the primary coil via the oil.

Owing to the rod-shaped core the diameter of the coil system may be comparatively small and the oil insulation `permits the space between the primary winding and the secondary winding to be minimized. Both facts are advantageous with a view to reducing the stray inductance, which is necessary for obtaining a high maximum coupling degree. However, the slight space (for example a few mm.) between primary and secondary `coil imposes high requirements on the accuracy of the guiding of the sliding movement of the primary coil. According to the invention these conditions are fullled by supporting the assemblage of core and winding by means of at least two members situated each at one side of the coil centre, viewed in a radial direction, and each of which is axially displaceable. i

In order that the invention may be readily carried into eifect, it will now be described in greater detail with reference to the accompanying drawings, given by way of example, and in which:

Figs. l and 2 show the coil systems according to the invention, Fig. l being a bottom view and Fig. 2 being a view in an axial direction.

Figs. 3 and 4 show in the same manner another example, of the invention, Fig. 4 showing it part in elevation, part in cross-section.

The coil system shown in Figs. l and 2 is supported by a horizontal, insulating bedplate 1 constituting the lid of an oil-filled container 3. The primary coil 5 and the secondary coil 7 (coupling coil) are secured to the bottom of the plate 1 and are immersed in the oil container 3, the adjusting device of the coil system being mainly mounted on top of the bedplate 1. The primary coil 5, together with a capacitor constitutes the oscillatory circuit of a high-frequency generator for heating purposes. The circuit capacitor and other generator parts may be mounted adjacent the coil system at the bottom side and upper side of the bedplate 1. All these parts may be of usual construction and for this reason they are not shown in the drawing.

The primary coil 5 is a helically wound copper tube, for example 6 mm. thick, which is supported by three insulating rods 9, 11, 13, for example made from hard paper, which rest against the coil. The ends of said rods are secured, for example by means of wedges, in two insulating end plates 15 and 17 (in Fig. 2 the plate 17 is partly broken away). Situated between the rods 9, 11 and 13 is a substantially cylindrical ferromagnetic core 19 of electrically poorly conductive cubic ferrite material having low losses (mixed crystals of iron oxide and other metal oxides). The core 19 has three long axial slots, into which the rods 9, 11, 13 are partly sunk, and a diameter of a few mm. smaller than the inside diameter of the helix 5.

The assemblage of helix 5, core 19, stay rods 9, 11, 13 and supporting plates 15, 17 is axially displaceable as a whole. To this end the two supporting plates 15 and 17 are interconnected by two metal slide bars 21 and 23 (see Fig. 2, in which mainly the fastening nuts of the bars are visible). These bars are movably supported in two flat trapezium-shaped insulating supports 25, 27 (Fig. 2) which also carry the coupling coil 7 (secondary coil).

This coil 7 mainly consists of a curved copper strip surrounding the coil 5 with a few mm. of intermediate space. The wi-dth of this strip, i. e. the axial length of the coupling coil, substantially corresponds to the length of coil 5; the core 19 is approximately 20% longer. Besides constituting the coupling turn 7 said strip also constitutes the double supply duct of this turn. The two ends 29, 31 of said strip extend parallel to each other with a few mm. of clearance space, obliquely upwards from the oil container 3 and are terminated by heavy clamps 33 and 35. These clamps together with the ends of the strip 7, 29, 31 are insulatingly screwed to a projecting bracket 37 welded to the container 3.

Water owing through the helix 5 is supplied through tubing 39 and 41 and is effective to cool not only the coil but also the surrounding oil which, in turn, cools the core i9. The heat due to the loss of energy in said parts is thus carried off safely. In order to cool the coupling coil 7, tubes 43, 45 are soldered to the copper strip 7, 29, 31 and connected, if desired in series with coil 5, to the cooling water duct.

Since the heat due to loss of energy in the core 19, which is still considerable despite the use of ferrite, is

mainly carried off through the coil S, it is vital that the coil support situated between said parts does not practically impede the flow of oil from the coil to the core and for this reason it is provided with large apertures sulficient for the passage of the oil. This is ensure-d by the construction with the rods 9, 11, 13 shown in the drawing. A coil support in the form of an insulating tube with large apertures in its wall is also serviceable for low powers. The small spacing (a few mm. or less) between the surface of the core 19 and the primary coil 5 is also contributive to the heat transference. Such a small spacing between two parts, between which a potential difference of, say 10,000 v. is set up, is possible owing to the insulating properties of the ferrite used.

Adjustment of the coupling between the primary coil S and the secondary coil 7 is performed by means of a handwheel 47, a roller 49 seated on its shaft and two metal bands 51, 53 adapted to be wound on said roller, this device permitting the primary coil to be displaced axially. For this purpose, one end of the band 51 is secured to the roller 49, the band further passing over a roller 57 freely movable in an aperture 55 of the plate 1, through the aperture 55 and along the bottom of the plate to the endplate 15, the band 53 similarly passing over a roller 59 and through an opening 61 to the end plate 17. The two bands are lightly made taut and on operating the handwheel 47 one of the ends is unwound from the roller 49, whereas the other band is wound up.

Since the clearance space (in an axial direction) between the primary coil and the secondary coil is only a few mm. high requirements are imposed on the guiding of the primary coil in connection with the considerable potential differences, for example 10,000 v., set up between the coils. These conditions are fulfilled by supporting the two end plates 15, 17, in the case under review by securing the slide bars 21, 23 to the two end plates, so that the coil is guided at both ends.

As is seen from Fig. l, this involves, however, that the primary coil, or at least its support, is required to protrude at both ends from the coupling coil 7 and this in all over a length equal to the maximum displacement of the coil. Hence, the overall length of the coil system will without special measures have to correspond to about three times the overall length of the primary winding.

A smaller overall length is obtainable by dividing the secondary coil, in the manner illustrated in Fig. 1, by a sawcut 63 at right angles to the axis. The two coupling turns thus formed cohere at one end, whereas the other end 65 of one half, as shown in Fig. l, is overlapped by a lateral extension 67 of the other half. By tightening a wing nut 69 at the upper side, the coil ends 65, 67 may arbitrarily be connected together by pressing them together.

For a minimum coupling degree the wing nut 69 is loosened and the coil 5 is moved outwards such that it is situated outside the right half (Fig. l) of the coil 7. The left half is then inactive so that the whole output current passes through the right half. This does not matter, since the output current is low in the case of a low coupling degree and consequently does not cause excessive heating of the operative right half. With a maximum or substantially maximum output current naturally both coil halves are operative.

Figs. 3 and 4 show a second form of the device according to the invention, whose construction mainly corresponds to that shown in Figs. 1 and 2. The guide for the primary coil 70, co-axial with the coupling coil 71, is in this instance obtained with the use of a single slide bar 73 of electrically nonconductive material, for example hard paper, of unround, for example rectangular cross-section, which bar extends substantially co-axially through the end plates 75, 77 and the core 79. For this purpose, the end plates 73, 77 are provided with suitable apertures and the core 79 has a spacious axial bore CII through which passes also one of the supply ducts 81 of the coil 70.

As is seen from Figures 3 and 4, the primary coil, inclusively of the supporting construction, has a smaller length than in the Figures 1 an-d 2, since in contradistinction thereto the end plate 77 is movable back and forth within the coupling coil 71 (preferably without contacting the coupling coil in connection with the high voltages). The structural length may be further reduced by making up the coupling coil 71, at variance with Fig. 3, of two electrically separated parts adapted to be connected in parallel, as shown in Fig. l.

In order to prevent the supply duct 81 attaining a comparatively high sellinductance, due to the surrounding ferromagnetic material of the core 79 which selfinductance would act as a leakage inductor and consequently diminish the coupling between the primary coil and the secondary coil, the core 79 is provided with a radial sawcut 83 which forms an interruption of the magnetic circuit present around the duct 81.

What is claimed is:

l. A coil system for high-frequency heating generators, comprising a support, an axially displaceable water-cooled primary cylinder coil in the form of a helix, a stationary coupling coil surrounding the former and including a metal strip curved to form a cylindrical shell, said coil system being mounted on said support and immersed in insulating oil, and a substantially rod-shaped ferromagnetic core of electrically poorly conductive cubic ferrite being contained in said primary coil, the length of the primary coil exceeding by more than twice the diameter and the support of the primary coil and the core being provided with apertures in order to permit unimpeded transfer of heat through the oil from the core to the primary winding.

2. A coil system for high-frequency heating generators as set forth in claim 1 further comprising at least two supporting members for said core and primary coil assembly, each of said supporting members being located on each side of the coil center thereby permitting said primary coil to be axially displaceable.

3. A coil system for high-frequency heating generators as set forth in claim 2 further comprising two insulating end plates and a frame therebetween, said primary coil being wound on said frame.

4. A coil system for high-frequency heating generators as set forth in claim 3 further comprising two spaced guide pieces, 'at least one rod movably supported therein, said rod interconnecting said end plates.

5. A coil system for highfrequency heating generators as set forth in claim 3 further comprising an insulating, non-circular rod extending in an axial direction through said coil and secured at its ends to said frame, said end plates being displaceable on said rod.

6. A coil system for high-frequency heating generators as set forth in claim l wherein the space between said primary coil and said coupling coil is less than 5% of the diameter of the coupling coil.

7. A coil system for high frequency heating generators as set forth in claim l wherein said coupling coil is divided by a transverse cut into two windings extending one behind the other in an axial direction, and a switch member for connecting said windings in parallel.

8. A coil system for high-frequency heating generators as set forth in claim 1 wherein said ferromagnetic core is provided with an axial bore.

9. A coil system for high-frequency heating generators as set forth in claim l wherein the spacing between the primary coil and said core is only a few mms.

References Cited in the file of this patent UNITED STATES PATENTS 

